1. Field of the Invention
The present invention relates to a chip-type piezoelectric filter comprising piezoelectric substrates, each having oscillating sections, and also to a filter circuit using such a piezoelectric filter.
2. Description of the Prior Art
Chip-type piezoelectric filters (or energy trapping type piezoelectric filters) are known and disclosed, inter alia, in Japanese Patent Kokai No. 3-97313 and Japanese Utility Model Publication No. 63-1460 and other patent documents.
Such a chip-type piezoelectric filter typically comprises a piezoelectric substrate, oppositely disposed input/output oscillation electrodes on one of the opposite surfaces of the piezoelectric substrate and a common oscillation electrode disposed on the other surface to cover said oscillation electrodes. Additionally, Japanese Patent Kokai Nos. 60-123122, 2-105712 and 4-4603 disclose an energy trapping type piezoelectric filter comprising a plurality of piezoelectric substrates that are arranged to form a multilayer structure. Various other arrangements of laying piezoelectric substrates to form a multilayer structure are also known. However, in any of these conventional multilayer structures, the electrodes disposed on the piezoelectric substrates are mutually connected according to a predetermined pattern to produce a single circuit.
In other words, they are not adapted to provide a multifunctional feature where one or more than one of the piezoelectric substrates are selectively used to make the piezoelectric filter operate for different functions or frequencies.
More specifically, a conventional piezoelectric filter typically comprises a single piezoelectric substrate and operates to allow only a given frequency to pass therethrough. Meanwhile, while various mobile telephone service systems including Portable Digital Cellular (PDC) Telephone System and Personal Handyphone System (PHS) are currently available, they are not compatible relative to each other and require users to use telephone sets particular to the respective service systems. Since each of these mobile telephone systems has its own advantages, there is a demand for a single portable telephone set that can be used for any of a number of different telephone service systems simply by selecting the desired system by means of a switch it has. Such a multi-applicability device may find a variety of applications other than the portable telephone set.
It is, therefore, an object of the present invention to provide a multi-function filter that meets such a demand, has a simple construction and is adapted to mass production.
Another object of the present invention is to provide a filter circuit using such a filter.